This invention relates to fins for a heat exchanger which have been treated to be hydrophilic.
Heat exchangers of various types have been used in a wide range of applications including room air conditioners, car air conditioners and air conditioners incorporating space coolers and heaters, for example. These heat exchangers are made preponderantly of aluminum and aluminum alloys. As illustrated in FIGS. 1 and 2, they generally comprise a zigzagging tube 1 for carrying a coolant, refrigerant or the like and a multiplicity of fins 2 disposed substantially in parallel to one another around the tube. In the diagrams, 2' denotes a protective plate.
When the surface temperature of the fins 2 and the coolant tube 1 falls below the dew point while the cooler is in operation, dew adheres to the surfaces of the fins and coolant tube. At times, the dew corrodes fins of aluminum or aluminum alloy, producing a white corrosion product (consisting of aluminum hydroxide and other compounds). The surfaces of the fins therefore normally are provided with a rustproofing layer, for example, by a chromate-treatment or, in recent years, a resin coat or a silicate coat.
To reduce size and improve performance, the designs for heat exchangers of this class of late have employed increasing numbers of fins and, therefore, have had an ever increasing available area of contact between the incoming air and the fins. For the same reasons, the space separating the fins is being reduced to the greatest extent possible without increasing the resistance to air flow between the fins.
When the rustproofing layer mentioned above is hydrophobic, the dew adhering to the fins collects into hemispheres or spheres, which may grow until they reach the adjacent fins. When the dew reaches to the adjacent fins in this fashion, it can continue to collect by capillary action, clogging the spaces between the fins, as illustrated in FIG. 3. This phenomenon is called bridging. In FIG. 3, reference numeral 3 denotes a dew bead which has developed the bridging phenomenon, and 3' two dew beads which have yet to reach this stage.
When the dew induces this bridging phenomenon, the resistance offered by the fins to the passing current of air increases notably, the heat-exchange ratio consequently is lowered and the cooling capacity of the heat exchanger degraded. The fins, therefore, should possess a hydrophilic surface.
The methods proposed to date for imparting a hydrophilic surface to the fins include forming thereon a coating containing a surfactant such as polyoxyethylene nonylphenyl ether on the surfaces of the fins, coating the surfaces of the fins with colloidal silica or water glass, and subjecting the surfaces of the fins to a post boehmite-treatment, for example. The coating containing the surfactant shows insufficient affinity for water and inevitably induces the bridging phenomenon. The coating of colloidal silica or water glass is so rigid that the press die and cutter used in fabricating the fins become seriously worn. Moreover, since this coat is as brittle as glass, the surfaces of the fins (particularly the surfaces of the flange portions) are liable to sustain cracks, fissures and the like during the course of fabrication. The trend toward such heavy wear and cracking is particularly conspicuous when the film is made of colloidal silica. Finally, the boehmite-treatment is not economical because of very high cost.